text:
Where are the microphones for the black box located in the cockpit?
All modern commercial airliners have a CVR (Cockpit Voice Recorder) on-board to record what the pilots are saying during the duration for a flight. I feel that the location of the microphones is important because they should be in a place that can easily pick up the cockpit conversations but should also be discreet enough to avoid accidental damage or even intentional sabotage by hijackers/suicidal pilots.
So where are the recording devices or microphones located in the cockpit for this?
Moreover:

Is there more than one microphone in the cockpit to provide stereo sound?
If so, do investigators listen to the recording from each microphone?
Are there any pictures of these microphones in the cockpit?

You could just switch off / trip the CVR if you wanted to sabotage the CVR. The flight investigators would probably work out you did, however. (See http://en.wikipedia.org/wiki/SilkAir_Flight_185#CVR_and_FDR_deactivation)

I am well aware of SilkAir 185, but thanks for sharing anyway :)

The headsets worn by the pilots have a microphone arm. These audio channels are split and routed to the black box.
Additionally, there is at least one microphone embedded in the cockpit, usually overhead.
The black box can support recording of up to 4 audio channels, so each channel can be examined separately.
Since the headsets and overhead microphone are inside the cockpit, stereo audio is available.
Can't find any pictures for you, sorry!
Seethis sources:
Avstop
FAA (pdf)

i don't know of any headset microphone that records in stereo; typically there are only two wires going out to the mic (and if there happen to be three, it's going to be an XLR configuration for noise reduction).  you could certainly create a stereo signal from a single microphone, but that would be a huge waste of a whole channel on the cvr.  stereo audio is only "available" if you do a little geospatial post-processing on the recorded signals, taking into account the locations of the microphones.

Each of the three crew gets its own array of mics including the hand mic, boom mic, mask mic, his speaker and headphones (the microphone he uses for radio communication and what he hears from the radio). The third crew member's channel may be connected to the PA.
The fourth required channel is for the cockpit area mic which records general sounds and alerts.
Source: NTSB

The Cockpit Area Microphone (CAM) can be in a variety of places due to different cockpit designs.  Generally, it is supposed to be installed in the best position for recording voice communications.
On a Boeing 777, it is on the overhead panel next to the passenger seat belt sign switch:

Image credit: Craig Murray / Airliners.net
On an Airbus A318/A319/A320/A321, it is in between the windscreen and the overhead panel, just below the exterior lights switches:

Image credit: Niksa Radicevic / Airliners.net

the 777 sure has a funky looking microphone! +1 for the nice pictures :)

authors: Huá dé ní 華得尼


text:
How many versions of the P-51 dorsal fin fillet are documented?
How many different versions of the P-51 Mustang DFF are there?

This seems somewhat broad; any number of P-51 tail variations any of us have seen will of course not include any we haven't, but that still exist. The only correct answer will involve a thorough audit of every batch of P-51s North American ever produced, which is not only way more work than could reasonably be expected for this site, it may well be impossible if there are no present-day examples of any given production run that had a particular dorsal strake variation.

I'll give it a shot though... :)

A quick search of Google Images seems to show only three major variations on the tail for production P-51 variants:
Razorback (all P-51A, B, and C variants; no dorsal strake and a higher rear fuselage line):

Bubble canopy without the strake (seen on early D-variants)

With the strake (seen on later production D variants, integrated fully into K and H models and retrofitted to earlier Ds, sometimes to the Razorback B/Cs):

There is some difference in the prominence and exact shape of the fin fillet as the design continued to be developed; for all B/C/D variants it was a thin piece essentially bolted onto the strakeless frame. For the H variant in the bottom picture, it was integrated more fully into the fairing of the vertical stabilizer.
There's one more I've seen and I'm not sure if there's just the one example, but there is a P-51 "XR" variant, "Precious Metal", with dual counter-rotating props and further aerodynamic refinement, including a fairing behind the modified bubble canopy that narrows into a continuous strake along the aircraft's rear spine:

Other custom racing variants have existed over the years, all heavily modified from USAF production variants.

Notice that I answered my OWN question about 4 years after posting it...I've learned a lot since then! :)

The H model has a larger, more prominent tail -- but keeps the dorsal fin.  This site https://www.mustangsmustangs.net/p-51/variants does a nice job going through variants.  The P-82 appears to have the biggest dorsal fin of them all!

Pictured is the P-51D of my late father in law, Captain Richard W. Dunkin, 325th FG, "Checkertails".  His P-51 "Thisizit" did not have the tail fillet as illustrated above in other P-51Ds.

The second photo in the other answer already shows a P-51D without the fillet.

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text:
Which engine(s) do airliners taxi with?
I  have read that pilots only taxi on 1 engine to save fuel.  Which engine does a twin engine jet such as a 737, 777, a320 or similar use to taxi? Left or right, or does it vary by pilot or some other factor?
On 4 engine jets such as a 747 or a380 pilots use 2 engines. I assume its a matching pair, inner or outer.  But again, which pair?

It's completely up to the airline, what engine and how many are used to taxi.

Possible duplicate: Taxiing with one engine: Is engine #1 always used or do they switch?

@mins out of interest, if you found the duplicate why didnt you enter a close vote?

@Jamiec: The OP could be interested in Embraer, Boeing, Bombardier, in addition of Airbus and its specific PTU.

I believe that it depends on the airline's procedure whether or not to shut down an engine for taxi.
I think 4 engine jets use one on each side but for 2 engine jets the engine chosen has no impact and thus the one chosen is not very relevant.

The chosen one is very relevant since each engine powers different hydraulic circuits.

Interesting historical note to the answers already listed is that the Boeing 707 could only shut down #2, #3 and #4 engines for whatever purpose without availability of a ground air-start unit (huffer). The #1 engine nacelle has a "bump" on top which is the compressor to start the other engines. If ground-start air wasn't available then the #1 engine had to be kept running.

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text:
777-300ER venting something from left engine outboard side - what is it?
Crossposting from Metabunk with permission from the thread opener marko.posavec.

Their photo shows British Airways Boeing 777-300ER G-STBF in flight with something producing a contrail or the like from the outboard side of #1 engine.
What is/could be causing this?

It might simply be that the de-icing is on and that is melting ice from the cowl's lips

Ice and de-ice at 36000 ft?

Proof positive that the chemtrail phenomenon is real :;

Wild hypothesis: some sort of un-evenness in cowl / nacelle that is causing turbulence and triggering condensation?

@CptReynolds only a theory, just like all the other ones

Cowls are are evaporatively anti-iced. They are kept above the boiling point so supercooled waterdroplets turn to steam right away on contact. If ice formation is allowed, it can damage blades.

Well, it's not condensate water from the tail pipe, and it looks to be originating from the main cowl, so it can only be one of the Big Three fluids; fuel, engine oil, or hydraulic oil (for the reversers, assuming they're hydraulically driven). Such leaks are not unheard of.
If I was flying 2000 ft below them on the same track so that I was able to observe that for a while, I'd probably contact ATC to relay to them my observations.

This was taken when the aircraft crossed Romania inbound LHR from BOM, so it probably had been ongoing for a while, or would it start mid-flight? In any case, the flight didn't divert and the aircraft remained in service for another flight about 3 hrs after landing. So at least it wasn't as dire as the Air Transat one.

re "so it can only be one of the Big Three fluids" -- consider https://aviation.stackexchange.com/questions/100213/777-300er-venting-something-from-left-engine-outboard-side-what-is-it#comment279711_100213

Yes of course. Does anyone have the QRH for these three leaks at their fingertips? I'm cut off from mine for another couple of days... probably not far off from "land nearest suitable" for some of these, right?

Possibly a leak could make a noticeable contrail and be too small to even notice fuel quantity wise and go undetected for a while. Or possibly it's just a fluid like engine oil venting off for some reason.

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text:
Could D.B. Cooper know where he was landing, when he parachuted?
The background story is a hijacking that happened 1971.
The hijacker, referred to as D.B. Cooper, jumped from a Boeing 727 using the aft airstair. He knew in what air corridor he was (Victor 23, heading south to Mexico from Seattle) and the time.  The mount St. Helens (8,365'), was about 10 miles north to him. Portland was 20 miles south. He had however zero visibility of the ground. It was nighttime and raining heavily. I suppose he would not see any lights from Ariel (WA), 10 miles north to his position, and which has a population of about 700 nowadays.
With what precision could D.B. Cooper know where he would fall?
There is another related question, but not a duplicate here: Would a 90 kg object jettisoned from an aircraft create a detectable movement?

I'm guessing if he was good on charting, had an accurate weather report, knowledge of course planning, and kept good time he could know where he was within a few miles.

nope. maybe within 300 miles or so

Really really depends how familiar he was with the area: if you are e.g. a glider pilot familiar with a certain area you`d know where you are just looking at mountain tops out of the clouds. Then with a compass you could walk in the correct direction to reach a major street.

According to en.WP, the investigation later determined the path of Flight 305 had been farther E. than its pilots believed and the drop zone was appreciably farther SSE than calculated, in the drainage area of the Washougal River. https://en.wikipedia.org/wiki/D._B._Cooper#Later_developments

We have no idea how much he did know, but it's possible he had a basic idea. The fact he came up with the plan at all suggests some kind of aviation background (the air stair, for example, wouldn't occur to many people): so we can probably assume at least some basic navigation ability, especially with how thought out the plan appears to be.
He knew where the aircraft was starting from. He knew the airspeed it would travel at (100 knots, just above stall speed), which would give him a rough idea of how far it had travelled – particularly if he had information about the wind at that level.
Distance = Time x Speed... so all we're missing is the Time. If he knows how long they've been travelling, that's sufficient to allow him to have a reasonable idea of where they are along a known track, particularly if he carried a compass.
It's hard to be sure without the precise weather conditions available, but even in rain then as long as you're below the cloud layer, I wouldn't be surprised if Cooper could see the lights of either Portland or, more likely (considering the angle) Ariel: towns are pretty visible from the air. For example this photo is London from 10,000ft. Admittedly London is quite a lot bigger than Ariel, and the weather is clear, but considering the "closer" areas of the picture are a couple of miles, and the "distant" areas are more like 20 miles away, it's fairly easy to see how even a small town can be quite visible.

(source: https://www.flickr.com/photos/andygocher/12836895204)
Note that at 100 knots ground speed the aircraft is travelling at ~2 miles per minute across the ground. I can't find any wind data, but considering the 8:13 jump time was only approximate, it's more than possible that he saw the town below and a mile or two below and behind the aircraft, took a minute or so to prepare, and then jumped when 4-5 miles from the town.
He probably didn't know precisely where he was, but there's a reasonable chance that he saw a town below and that was sufficient for him to attempt the jump. He had the equivalent of over a million dollars in his pocket, he only had to find a town and get on a train in order to escape.
We also have no idea whether he had a chart or similar in his bag, nor whether there was any potential visibility of Mount St Helens (possible, considering the 1/4 moon that night). It's unlikely at 10 miles, but they'd passed it earlier and he may have used it as a point of reference.
In short: he probably had a rough idea of the region (to within 50 miles or so) and quite possibly had just seen a small town.
Did he know what town it was? Probably not, but he likely didn't care either: he had enough money that as long as he survived the jump and walk into the town, it probably didn't matter what the town was.

"[H]e only had to find a town and get on a train in order to escape." Yeah, that's not really how the US passenger rail system works (or worked in 1971). If he landed near Ariel, he'd be very fortunate -- only a 20-mile hike to Longview or Vancouver!

Also, Wikipedia refers to "extremely limited visibility and cloud cover obscuring any ground lighting below", so I don't think the general assumptions of this answer correspond to the actual situation.

While it's impossible for anyone to know for sure it's unlikely he would have had much idea at all of where he was. According to the articles I have read when he jumped it was at night, in heavy rain, and the ground was obscured by cloud cover. The visibility was bad enough that neither of the F-106 pilots which were trailing the airplane saw him jump from the airplane.
He did not direct a specific course, what was agreed was that they would fly to reno to refuel, and then to Mexico. The gear was to be extended and the flaps at 15 degrees, so they could not fly that fast.
Pilots and flight attendant were in the cockpit with the door closed, all Cooper would have had for reference is the outside windows and the view out the back, plus any instruments he brought with him. He didn't have a current weather brief, so no knowledge of the winds aloft and how that would affect ground speed. Given the conditions he would not have been able to see much out of the window at all.
You have to consider human factors as well. The flight was at 10,000ft, unpressurized, and Cooper had been under stress without rest for hours. He had been drinking moderate amounts of alcohol intermittently. All this would have degraded his ability to determine location.
The lack of ground speed data, and exact course meant he would only have a vague idea of his track over the ground, and the poor weather and darkness would have prevented him from using landmarks. He would have had an approximate idea of where he was - somewhere between Seattle and Portland, but other than that he was literally in the dark. He may have spotted the glow of lights from a town and made his move, or he may have deliberately jumped in an empty area to escape. I don't think we will ever know.


text:
Why do the N1 and N2 fans have different shafts?
This question here contains a cutaway of a jet engine (which I'm putting right....here)

Credit: K. Aainsqatsi, Wikipedia
After reading the answer, and looking over the chart, I noticed that the shafts for the N1 and N2 fans are completely separate (something I'd never noticed before.)
What purpose does that serve?  Do they always move freely from each other, or can they be locked?

They are never locked, the purpose is to have two different rotation speeds to optimize the low pressure compressor (lower airspeed) and the high pressure compressor (higher speed). RR adds a third shaft to rotate the fan (extra low speed).

@mins So they have an N1, N2 and N3 on their designs?

related: https://aviation.stackexchange.com/q/25916/1467 https://aviation.stackexchange.com/q/20733/1467

Does the center portion of the spool, the part inward of the bypass, act as part of the first compressor stage?

@mins: There are many more three-spool turbines than only the RR Trent.

Several reasons:
The fan, LPC/T, and HPC/T spools all operate at different speeds for optimal efficiency.  A hard link between them affects the speeds at which the other spools can operate, reducing efficiency.
Separate spools reduced the rotational inertia of each, reducing 'spool-up' lag, and allowing them quicker response times to throttle inputs.
The spools can be contra rotating, reducing the effective moment about the engine casing and mount, making the engine lighter.

I wonder if you could maybe add a bit more detail?  I like what I'm seeing I just...want to know more I guess


text:
Why do jet engines smoke?

(YouTube video)
From the video linked above, I was wondering why the engines emit so much smoke? Has something gone wrong? Poor maintenance? Isn't it dangerous?

Because they think it looks cool?

Yeah, they are dangerous,  hard and rugged, steeped in 1950's values and aesthetics, and know about their lifetime being finite anyway ... what do you expect?

Smoke in jet engines is usually from unburnt or partially burnt fuel (or by water injection). Early jet engines used to create a lot of smoke due to these reasons (and due to lack of environmental regulations).

KC 135 engines creating smoke due to water injection By USAF Photographer - USAF photo, Public Domain, Link
Water injection, while cooling the engine, quenches the flames to an extent, resulting in unburnt fuel, which comes out as smoke. However, aircraft have moved away from water injection, reducing emissions.
Another reason for the smoke is inefficient combustion- older jet engines (turbojets in first aircraft and low bypass turbofans) were not as efficient as engines today, which premix the fuel with air and pre-vaporise it before combustion, rather than using droplet combustion- basically mixing fuel more evenly with air. Requirements of fuel efficiency along with stricter regulations mean that engines today have less emissions compared to their predecessors.

Smoke during Boeing 707 takeoff; image from nycaviation.com
In this case re-engining the aircraft with more efficient ones usually results in a reduction of smoke. For example, the USAF re-engined the KC-135s (from Pratt & Whitney J-57-P-59W turbojet to the CFM International CFM56 turbofan), which along with the elimination of water injection, reduced smoke.

On the top is KC-135s from an exercise in 1979; image from network54.com. Below that is KC-135Rs from Singapore Airforce; image from reddit.com

Oh my god... The amount of pollution there is awful. I'm glad we have more stringent environmental policies now

@JamEngulfer Mind you, those are all pictures from high-load scenarios - mostly take-off and landing. At cruise altitude, the emissions are much lower. That's why it took so long to get rid of the smoke - the extra efficiency loss from unburnt fuel on take-off/landing is relatively small on a typical route, while the technology to make the jet cleaner makes it more complex, and thus less reliable and more expensive. Making a clean jet engine is quite a challenge, and as always, a game of trade-offs.

@JamEngulfer There is always a chance that filters have been placed upon the photos to make the smoke more obvious that it was at the time. Monotone photos do this. That does not mean the effects were worse, simple that they are more obvious. The final image is the only marketing image. Visible emissions are potentially doctored out.

@Gusdor I agree that contrast enhancement of the b/w images might make the smoke look darker. However, it would be a lot of work to Photoshop out smoke in the last image, and would have very little propaganda/marketing value. The modern engines do produce a lot less smoke, and the last photo looks entirely plausible to me -- compare it to any other photo you choose of a modern jet engine in level flight.

The comparison between the last photo and the others is slightly unfair. All the smoky photos are taken at times of high load, which produces more emissions, and looking roughly along the long axis of the plane, which makes the smoke look denser. The non-smoky photo is taken at a time of low load and looking perpendicular to the long axis of the plane. Having said that, I don't dispute the conclusion, and there are plenty of photos of non-smoky take-offs of KC-135s with modern engines.

Fundamentally, because jet engines do not burn a pre-mixed fuel/air mixture.
Fuel can burn in three ratios: lean, stoichiometrically and rich; respectively: an excess of air, exactly enough air, and an excess of fuel. Ideally, one would always burn in a stoichiometric ratio; intuitively, one can feel that this means you don't waste fuel on heating an excess on air, and you're not left with any unburnt fuel. This is how a typical spark ignition engine works: fuel is mixed in the carburetor, compressed and burnt. The throttle varies how much fuel/air mixture enters the engine, but the mixture will always be (close to) stoichiometric.
A jet engine does not have the luxury of pre-mixing the air/fuel mixture before ignition. Spraying Jet-A in the compressed, (hot!) burning air will lead to ignition as soon as it comes into contact with oxygen, whether you like it or not. The problem is that it will already ignite when the local mixture is still rich. This will inevitably lead to soot formation, which is the smoke you see. This is especially prevalant at high power settings (like during takeoff) - at a given RPM, the amount of air that is pumped through the engine is constant, and the amount of power is varied by injecting more or less fuel into the engine, thereby creating a less lean mixture at high power. Note: the engine RPM will react to the power setting in a jet engine; however, the amount of air will generally lag the amount of fuel at higher power settings
There are some methods to mitigate soot formation, which are all based on increasing the fuel-air 'mixedness' before ignition. A lot of research is being done on this by improving the combustor. The linked Wikipedia article has a very nice and thorough explanation on what exactly is being done for this, but in general (and extremely simplified), it comes down to spreading the fuel as much as possible and introducing as much air as possible (but remember that running too lean reduces efficiency), all without dousing the flames. See the image below for the complex arrangments already made in the combuster on an older cannular design.

Source: wikimedia

Collin Krum has written a fairly in-depth article on the matter for Jalopnik. It's also worth noting that not all of the engines in that video are jet engines. From the article:

Low bypass engines aren’t as efficient as high bypass engines, but
  water injection is the technology that is most responsible for the
  seemingly-eerie pictures of older airplanes riding black columns of
  smoke into the sky.
...
Because the engine core is cooled by the injected water, the
  combustion chambers aren’t able to burn all of the fuel and water
  mixture, so some particles of the fuel and water are vented out the
  engine, which materializes in the form of the characteristic black
  smoke.

Some of the AGE/GSE I work with uses jet engines. During startup and shutdown, surely expect the temperature not to be at its maximum. That can be one cause of unburnt fuel. Another cause could be a gradual buildup of fuel depending on the interval of the pneumatic thermometer's ability to help quench the bleed air.


text:
Did the Douglas SBD Dauntless have a second set of controls?
In the book Never Call me a Hero, the author mentions that there were controls in the rear seat of the Douglas SBD Dauntless. However, I don't recall ever hearing this before, though I know the TBD had controls in the second seat. Did the SBD have controls in the rear seat?

The Dauntless did have controls for the rear gunner- a pair of permanently attached rudder pedals and a control stick that was stowed away on the port side until needed. See:
https://www.tapatalk.com/groups/warships1discussionboards/sbd-dauntless-flyable-from-rear-cockpit-t2435.html
Juan Jimenez is correct that it would be very difficult to operate, what with the armour plate for the pilot and the radios right in front of the gunner. See here for a story of flying a veteran Dauntless where the pilot gets a brief familiarisation flight from the gunner's seat (it's mentioned in the fourth paragraph after "SBD Glider: Engine Out"):
https://www.aopa.org/news-and-media/all-news/2011/january/pilot/flying-a-national-treasure-the-sbd-dauntless
You can see the gunner's control view at the Smithsonian's walk-around view of the Dauntless, it's images four and seven in the collection (six shows the pilot's view and eight the gunner's real place of business):
https://airandspace.si.edu/collection-objects/douglas-sbd-6-dauntless
Photo seven

I stand corrected, and I would hate to be the poor soul stuck with trying to land that on a carrier with a disabled or dead pilot up front.

Agreed, I think the limit to flying from the rear seat would be "roughly straight and vaguely level" for as long as it takes the pilot to finish scribbling on his maps or until the plane was in a safe-ish place for a bail-out.

No kidding. There is no panel back there! No instruments, no navigation, nothing. i wonder if the gunner could even use the radio! That's nuts.

Also: Very nice for a first answer! Welcome!

@JuanJimenez Even without a radio, a few warning shots above the tower should be sufficient for landing clearance.

https://lifehacks.stackexchange.com/questions/20951/how-does-a-blind-passenger-not-die-if-driver-becomes-unconscious

Interestingly, I came across a story of actual use of the rear gunner flying the aircraft because the pilot was wounded / disoriented, during the Guadalcanal campaign. It was even during a night flight.  It's in the book The Battle for Hell's Island.

The concept of the rear controls (stick and rudder pedals) was two fold in that

the gunner/radio operator could recover the aircraft in a nose up climb attitude if/when the pilot blacked out from excessive G  during a dive

and

the aircraft could be recovered to straight and level should the pilot be incapacitated allowing the rear seater to bail out. The rear controls were never intended to land this aircraft. Interestingly the Germans incorporated an auto recovery capability into their Stukas in the early 1930s, this technology was never adopted by US manufacturers

Wouldn't the gunner black out from excessive G before the pilot? The pilot is causing it, therefore he's anticipating it and that should slightly increase his resistance to it even if back then they didn't know the right tricks for it.

The gunner always controlled the radio but he had no access to tail hook, engine control beyond throttle or anything else much needed to land safely on a ship.

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text:
Why is it inefficient to expel high speed gas?
I'm reading the wikipedia page on turbofans, where it says turbojets are inefficient at low speeds because:

energy is wasted as the propelling jet is going much faster rearwards than the aircraft is going forwards, leaving a very fast wake. This wake contains kinetic energy that reflects the fuel used to produce it, rather than the fuel used to move the aircraft forwards.

Can someone please explain this a little bit?  Why is there a better way of utilizing kinetic energy than simply expelling the mass backward?
It seems to me like attempting to "capture" the expelled air is akin to putting an air-powered generator behind a turboprop propeller and using the energy to spin an extra propeller.  How is that possibly more efficient than just letting it be?

Umm just looking at the title alone I'd say that's more a question for Medical SE  : (

It depends on what you call efficient.
In principle, the force imparted on the airplane is equal to the momentum expelled backwards:
$$F=\frac{\Delta m}{\Delta t} \cdot v$$
The power requirement to deliver that force is equal to the kinetic energy in the exhaust,
$$P=0.5\frac{\Delta m}{\Delta t} \cdot v^2$$
So, because of the $v^2$, you obviously get more force for your energy input if you accelerate a lot of air by very little speed, instead of accelerating very little air by a lot.
This assumes that it doesn't cost energy to actually "get" all this air mass through your engine. In reality, friction losses start dominating at some point. For rockets, you need to actually bring all the mass up with you instead of just taking in some atmospheric air, so then it's actually more efficient to accelerate very little mass by a lot.

This also applies for example to helicopters, in which they build big rotors to accelerate large masses of air to slow speeds

So in a world without fan blades, could a plain jet or rocket benefit from mixing the rapidly moving expelled gas, with slower moving normal air?  Kind of like the way a "bladeless" dyson fan turns a small jet of fast air into a large jet of slower air.

Yes, this is called an air augmented rocket. https://en.m.wikipedia.org/wiki/Air-augmented_rocket

seems to me like attempting to "capture" the expelled air is akin to putting an air-powered generator behind a turboprop propeller to turn another propeller

You're close, but why not put the generator turbine directly behind the gasses being expelled from the engine?
This is how turbojets are designed.  One "generator" powers the compressor, another can be added to run a fan or prop.

This wake contains kinetic energy that reflects the fuel used to produce it, rather than fuel used to move the aircraft forward.

This is utter nonsense.  Burning fuel will never move an aircraft forward unless it is used to create some type of propulsive force.
The propulsive force per unit fuel burned is the efficiency factor.  Dumping a 55 gallon drum of fuel next to an airplane and burning it (obviously) will have a thrust efficiency factor of 0.
Burning it in a tube and allowing the heated gasses to expand backwards will created an equal amount of momentum, as mv, forward.
Burning it in a tube (or cylinder) and using the expanding gasses to create mechanical force to turn a fan or propeller increases efficiency further, but only under certain conditions.
Propellers are only more efficient up to around Mach 0.4, while (ducted) fans (extracting energy from the expelled gasses of the jet engine core) remain more efficient at higher Mach numbers because they can create more momentum per unit fuel burned than the turbojet.

"Propellers are only more efficient up to around Mach 0.4 ..." Is this why piston engines and turbo props fell out of favor for commercial passenger aviation, but why the "relatively slow" Lockheed C-130 still uses turbo props to this day?

@dgnuff  ...which gives the C-130 the ability to take-off on much shorter runways for a given power and weight, which is important for a military transport aircraft.

I think that the quoted comment about the wake being inefficient is a clumsy attempt at saying that turbulent air represents wasted energy?

@MikeB   The "wake" in this case is expelled engine gasses.  These plumes become turbulent, but not at the exhaust (if the pipe is properly designed).  Sanchises explains the physics mathematicly.  I marveled at the logic, as fuel is burned for propulsion, and propulsion moves the plane forward (with varying degrees of efficiency).

@MikeB, Put another way, the turbulence doesn't steal energy from the engine. It's more like, proof that the engine allowed the energy go out the tail pipe without using it to do any useful work.


text:
What kind of jet plane is this?
The cockpit can hold one person, is smaller than a compact car and as narrow as a motorcycle. What kind of plane is this and how hard would it be to fly?

In its current state, flying it would be extremely difficult! ;)

On the contrary, that plane is VERY unlikely to crash any time soon.

There is a retro simulation of it released by Microprose called AcroJet https://en.wikipedia.org/wiki/Acrojet

one of the most underrated aircraft I know.

Go to Oshkosh (Air-Venture). You'll see several of these in flying condition. There's also one in Udvar-Hazy (the OTHER Smithsonian Air and Space museum south of the Dulles airport in DC).
The original BD-5 was a kit-built, pusher-prop (wooden, fixed pitch), single-seater, retractable tricycle landing gear, aircraft. More recently, others have bought licenses to the design and adapted them to jet engines. It's a high performance, tiny aircraft. You can buy one:
https://bd-micro.com
The problem of the original BD-5 was that it was designed around a collection of engine specs that didn't belong to any existing engine. The builders have had to find an appropriate engine and adapt it to the airframe. Jim Bede, the designer, tried a lot of different engines and never found one to include in the kit.
The one in the Smithsonian (Udvar-Hazy):
https://airandspace.si.edu/collection-objects/bede-bd-5b
It took over 5,000 hours for two guys to build. They flew it for nine hours and then gave it to the museum, likely because it scared them too much to fly it, and as builders, they would be liable for any crashes if they sold it.
It was a dangerous aircraft to fly. Fighter pilots loved it because the controls are familiar to them in that small movements of the stick result in large changes in the aircraft's attitude. The controls are extremely sensitive.
The safe alternative for a builder is to contribute it to a museum under the strict condition that it never be flown again, get a tax break for your contribution, and let thousands of people a day admire your pretty little airplane.

Would it be cheap to maintain it because it is smaller?

@Muze you'd need less paint, that'd help. And cheaper to construct because you need less materials. Probably cheap to run because it uses less fuel than a larger model too.

It was a dangerous aircraft to fly.  This is the biggest understatement on the page.  I think the BD-5J has a crash and pilot loss record that would probably rival some WWI fighters.

BD Micro kits don't take nearly as long to build because they have been reengineered. BD-5's also get bought and sold all the time, same as any other experimental aircraft. It's called a liability release and no experimental is sold without one, regardless of how safe people think it is. BD-5's are not inherently dangerous. That's been proven by pilots who have collectively accumulated thousands of hours in the type. They are unforgiving of pilots who do not do the homework and prepare themselves to fly a high performance aircraft.

The only reason BD-5J's have a high accident record is because of pilots who are not qualified to fly it, and because only a handful have ever been built. The aircraft requires pilots with enough experience to stay ahead of the aircraft. There are BD-5J pilots who have been flying the aircraft continuously for more than 30 years, including the pilots who flew the jet in Octopussy.

BTW, it should be noted that the UK CAA thinks all UK pilots are unqualified to fly the BD-5, prop or jet version, and will refuse to issue you a permit to fly for the type. http://www.aero-news.net/index.cfm?do=main.textpost&id=3a581247-39b4-4219-8ec1-0cb56d00f6cf However, if you go next door to Ireland they will be happy to issue you one. Go figure...

@JuanJimenez You make a good point, to be fair.  With that said, if an airframe is so touchy that a gross percentage of what would otherwise be considered fully qualified pilots can't grok the envelope before tasting dirt then... well, it's maybe a dangerous aircraft to try to fly.

@J... And there's the catch. Too many pilots think that because it is small and inexpensive, that makes them fully qualified. Unfortunately, the ones that bite the dust are most often not even remotely qualified.

The other thing to consider is that the vast, vast majority of BD-5 accidents have been caused by pilot error -- specifically, the fact that the high line of thrust means the nose will go up on loss of power. Because of all the engine issues with the design, many people rolled their own engine installs, which resulted in many engine failures on or soon after takeoff. Combine that with lack of currency during the build and thinking it will fly like a Cessna spam can, and there you have it.

That is a Bede BD-5, a microjet famously flown by James Bond. Based on the tail it may be the propeller variant.

(source)

Anecdote: the aircraft can also be admired in the London Film Museum at their Bond In Motion exhibition. In Octopussy (1983) Bond flew such a thing.

That is a very poorly-built example of a Bede BD-5B. I can tell by simply looking at the placement of the rivets. To me that is the #1 indicator of low build quality on a BD-5. The low quality of the build is probably why it's sitting on a pole instead of flying. The lack of a thrust attenuator makes it a prop version.
To fly the prop version you only need a private pilot's license. To fly the jet version you need an experimental type rating, which you won't get without convincing the examiner that you have enough experience to rate the privilege.
The prop version is relatively easy and inexpensive to maintain. The jet version is expensive to operate and maintain, mostly because of the cost of maintaining the jet engine and paying for fuel, insurance, etc.

This answer partially answers the original question, but includes now irrelevant information as the question was rolled back to the original. I advise you edit this answer to just contain the first part - the identification.

If I were to chase down all edits users make to their questions so i can edit my answers I wouldn't get anything else done. The additional information adds value. I will leave it as it. Thanks.

This, of course, and as many have noted here, is the BD-5, of James Bond "Octopussy" fame, for one, in which stunt pilot Corky Fornof flew THROUGH a hangar that the doors were being closed on. Someone has also posted the pertinent clip from the movie here. Not only that, he did it EIGHT TIMES, in order to get the proper shot! See:
https://www.theledger.com/article/LK/20090425/News/608138258/LL

The movie makes it seem like that's what Corkey did, but the man is crazy, not stupid. There is a difference. The doors closing were added to the scene after it was filmed and the final edit to the stunt was done. The interior shots were done with a static airframe bolted to a pole that was itself bolted to a convertible Jaguar whose tranny gave up the ghost after the filming. If you pause the video at the right time, you can see the pole. You can also see that if you rewind a bit, the approaching aircraft is a twin engine business jet.


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What is the cause of unusually strong wake turbulence from the 757?
The Boeing 757 (at least in the US) is in a special class of its own with respect to air traffic control wake turbulence advisories and separation. This is apparently due to it producing stronger wake vortices than would be expected from an aircraft of similar size and weight. What about the 757's design causes these strong vortices? What led to the decision to treat it differently than similar aircraft?

My understanding, primarily based on this archived NASA document (also linked to on the B757 Wikipedia article), is that there were several issues leading to the decision to make the 757 a special case.
First, there were some notable accidents where witnesses seemed to indicate the cause was 757 wake turbulence.
Cessna Citation - December 18, 1992 - Billings, Montana

Witnesses reported that the airplane suddenly and rapidly rolled left and then contacted the ground while in a near-vertical dive. Recorded ATC radar data showed that, at the point of upset, the Citation was about 2.8 nmi behind a Boeing 757 and on a flight path that was about 300 ft below the flight path of the 757.

IAI Westwind - December 15, 1993 - Santa Ana, California

witnesses reported that the airplane rolled abruptly and that the onset of the event was sudden. The Westwind was about 2.1 nmi behind a Boeing 757 and on a flight path that was about 400 ft below the flight path of the 757.

Additional accidents were noted with a Cessna 182, MD-88, and a Boeing 737.
Second, there were some controversial NOAA flyby tests which showed:

for a peculiar set of weather conditions that lasted about 0.5 hour, the vortex velocity of the 757 was approximately 50 percent higher than that of the 767 at similar vortex ages (younger than 60 sec) measured in less favorable weather conditions. However, the results also showed that overall the wake of the 757 decayed faster than that of the 767; in fact, the wake behaved as would be expected for an aircraft of the size and weight of the 757.

Third, they felt the slower approach speed of the 757 (vs. similar aircraft) made it more likely that trailing aircraft would inadvertently close the separation gap during approach.

Ultimately, I believe the correct answer to your question is actually that there is nothing particularly special about the B757 in regards to wake turbulence. This Air&Space Magazine Article indicates that

Boeing maintains that there is nothing about the wing design that would cause wake turbulence.

and

"The wake from the 757 is no stronger or weaker than you would predict from the weight, span, and speed of the airplane," agrees NASA's David Hinton, who has taken wake measurements at airports around the country and is principal investigator of NASA's NextGen Airportal Project.

The decision to put the 757 in its own category was controversial at the time and not based on particularly strong evidence. It was done out of precaution due to anecdotal evidence and possibly flawed test results. On the other hand, you could argue that ATC erring on the side of caution is best if there is any doubt. To be fair, a 757 is on the larger side of the small category.

if I recall, the 57 wake turb concern was specifically for when trailing edge flaps were extended, even partially. Going off of memory from when I worked in Seattle but believe there were some insider comments about the unique attributes of the flap geometry relative to the 767 and other airliners in that the t.e. flaps almost form a separate continuous, high-aspect ratio 'wing'. That is, conjecture was a significant trailing vortex from the outboard edge of the most outboard trailing edge flaps.

I believe you meant "large" instead of "small."

I have also heard that because the 757 and 767 were developed in conjunction the 757 (although a narrow body) shares much if not all of its wing design with the wide body 767. Due to the fact that most of the wake turbulence (vortices) of an aircraft in flight is due to the air's interaction with the wing, the 757 essentially using a heavy aircraft's wing (767) causes it to have an unusual amount of wake turbulence compared to its size.


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Seating distribution in passenger aircraft
Recently I was checking in to a flight and was asked if I'd like a window or aisle seat as usual and choose a window seat. I was then told that there are no more window seats available but I could get an aisle seat without someone sitting next to me and then just take that window seat. The plane was an ATR-72 so the rows were 2+2 seats.
I know about weight distribution to the front/back but I couldn't come up for a good reason to do this. What could be the reason for not giving me that apparently free window seat right away?

Depending on the airline they may charge more for a window seat so couldn't print a boarding pass for that seat unless you paid...

@Lnafziger good idea but that is definitely not the case. I'm a frequent flyer on that route and had window seats before without paying extra.

Could also have been someone was booked into that seat but the gate knew they were going to misconnect.

@casey hmm, plane wasn't even half full so mine wasn't the only free row. I doubt so many people missed the flight. Just remembered that I had a similar case before. Only that they didn't tell me I was allowed to just seat at the window. I just did ;-)

I think you're unlikely to get a good answer here. Whether you sit in an aisle or window seat has no practical implication for the aircraft. The answer would be purely related to the airline's boarding/seating policy. Even if you named the airline in question, there probably isn't someone here who would be an expert on their policies.

@BretCopeland yeah, I expected that much. Thought it was worth a try. If I don't forget I'll ask on my way back on friday.

Did you remember to ask? :)

@DannyBeckett they had some computer problems and there was a huge line so I didn't ask. But it won't be the last time I took that flight so there'll be another oppertunity :-)

Hey everyone, I almost forgot this was still an open question. Sadly the airline changed shortly after this and I didn't ask before :-(

One guess is that the flight was weight restricted and so it had to go out with fewer passengers than seats.  (This is more likely on a small aircraft.)  Their system may have enforced this by requiring that certain specific seats be blocked and not assigned to passengers.  If 5A was such a blocked seat, they couldn't give it to you, but they could give you 5B.  And then, once the flight has boarded, nobody will stop you from sliding over into 5A.

also (though this may not be the case with the ATR) there are seating arrangements in which specific seats don't exist. Think a window seat next to an emergency exit that can't be placed because of the slide sticking out into the cabin.

Too Localised, we can;t comment on the justification of one airline.

This question appears to be off-topic because it is about seating policies of a specific flight/plane/airline.

I don't think the question is off topic, just badly posed.  The real question is if they restricted where he could sit during take-off maybe for weight reasons.  Which, I'm quite certain, we have answered in another question somewhere...

@JayCarr do you have a good proposal on how to rephrase my question?

@NateEldredge best possible explanation yet. Too bad it's still just a guess.

@AndréStannek Well, with how you currently state your question you kind of dismiss the weight distribution answer.  I'd probably just edit it so that it says something more like "could this have to do with weight distribution"...  It's really the only question we can probably answer.

@AndréStannek And if that is your question, this may answer it: http://aviation.stackexchange.com/questions/1659/passenger-weight-distribution-on-a-commercial-flight

@JayCarr I know about weight distribution but to my knowledge it is only relevant along the longitudinal axis of a plane. That's why I dismissed it as an explanation. So the real question is "What other reasons are there?" which I pretty much asked. I think it is a valid question. The problem seems to be that no one has an answer to it. Assuming it to be too localized because of that seems wrong. Anyway, closing it as too localized might still be valid if someone KNOWS that this is a very strange and specific issue. This would also kind of answer my question, although not satisfyingly ;-)

There are certain restrictions due to which the airliner didn't give you the seat even if you were ready to pay extra for it (Depends on the operations policy by the manufacturer and the aircraft).
It's mostly the weight restrictions for the flight to take-off as there might be some obstacle for the flight during take-off(hoarding on the end of runway, climate etc.). If the weight is too much then it will result in a disaster so equal weight distribution is mandatory.
Moreover in the end it's airliner which has to suffer because the seat occupancy get's affected and it has huge losses incurred by not leaving to its capacity.

@Andre i wrote this post because i happen to read a news wherein the DGCA asked the Mumbai AAI to remove the banners from the runway exit(don't kow exactly which runway).IT was that an Airline by the name Indian Airlines requested the DGCA to remove some advertisement banners as it was having weight issues during take off(and also reducing the passenger count)

The only plausible reason is that the airline charges a different rate for the window seats, and you didn't purchase a ticket at the proper price point to get it.  It has no effect on the airplane whatsoever.

As I said in the comments that's is not the case.

It's how the computer system does it then, perhaps it blocks window seats until aisle seats are filled.  There is no operational reason for it.


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How do airlines pay for aviation fuel and services at an airport they don't normally use?
On flights of a regular nature to set destination airports, I assume that there is a system is in place between airlines and airports/providers to pay invoices for fuel/services.
What do airlines do if diverted to an airport that the airline never normally serves or a freight airline doing a one-off flight to an airport? How do they pay? Does the captain carry a company credit card to pay for these one off charges?

In the United States most places take credit cards.
Large carriers generally get billed, or for smaller airports they use a company credit card. There are even specialized credit cards that are designed specifically for this use.
There is also what is called "contract fuel". Basically there is a company that is a middleman, a fuel supplier who pays the airport fuel company and then rebills the airline. The pilot has a card that identifies them as a client of the fuel contractor.
For example, check out AEG Fuels, a typical fuel contractor. (Please click on that link. I get 25 cents every time you do. Just kidding.)

do they get air miles!

You've gotta check out this article!

Delta seems to be paying at least part of their fuel purchases using a corporate American Express card, and with quite the spending limit at that (from an SEC filing):

We have a purchasing card with American Express for the purpose of buying jet fuel and crude oil. The card currently carries a maximum credit limit of \$800 million and must be paid monthly. At December 31, 2016 and December 31, 2015, we had \$432 million and \$221 million, respectively, outstanding on this purchasing card, which was classified as a fuel card obligation within other accrued liabilities and a financing activity in our Consolidated Statements of Cash Flows.

This amounts to a line of credit of over a billion USD as of 2024 (current numbers here) that they seem to be making good use of.
Now their captains are most likely not carrying that card in physical form (the card is probably registered/"on file" with some service provider and billed as required), but I don't know the details.
I also don't know whether this is a common arrangement, or whether this is something unique to Delta – I could imagine so, especially given that Amex also operates their rewards credit card program.

Credit cards, wire transfer. In one case a crew even held a collection among the passengers after a diversion because they needed fuel and the airport fuel truck refused to accept anything except cash! (that however is exceedingly rare in civilised countries).
Or a simple call to the head office may get a purchase order for fuel sent to the relevant subsidiary of the company's regular supplier, probably the most optimal solution.

It could be that they usually accept other forms of payment but the airline has bad credit and they didn’t want to take the risk. There are also common stories of airlines on the brink of bankruptcy which were forced to suspend operations because suppliers would no longer provide fuel or services.


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Is it possible to fly horizontally with zero angle of attack?
If an aircraft has a zero angle of attack and the wing doesn't, is it possible to fly horizontally? Because as the book says, there is no lift in this condition,
Is this zero angle of attack of the airframe or the aerofoil?
Can you give a simple explanation?

depending on the airfoil profile (non-symmetric cambered ones) you can have lift also with 0 AoA

What book? I'm pretty sure that's wrong. Also the wing angle AoA is what's important for lift, not the aircraft AoA.

Unless you tell us which book you read this in (and what exactly it said), it is not possible to answer your question (the answer depends on whether the zero angle of attack is that of the airframe or the airfoil (wing)). We can try for a general answer though.
As far as angle of attack is concerned, there are two kinds- geometric and absolute. First, angle of attack is the angle between some reference line and the freestream (i.e. the velocity vector). The selection of reference line determines the 'kind' of angle of attack.
Geometric angle of attack, $\alpha$ is the angle between the chord and freestream; this is the one usually used. Absolute angle of attack, $\alpha_a$ is the angle between the freestream and the zero lift line (the line, which when parallel to the freestream, produces no lift in the wing).
The geometric and absolute angles of attack can be related through $\alpha_a = \alpha - \alpha_{L=0}$, where $\alpha_{L=0}$ is the angle between the chord and zero lift line.

Relation between geometric and absolute angles of attack, image from gtae6343.wikia.com
For a cambered airfoil, the zero lift line is negative i.e. the airfoil will produce lift even when the geometric angle of attack is zero (in this condition, the absolute angle of attack is positive). For a symmetric airfoil, of course, the zero lift line coincides with the chord and as such, the airfoil produces no lift when the geometric (or absolute) angle of attack is zero.
As far as the lift is concerned, what matters is the absolute angle of attack, while we usually use the geometric one. Simply put, what matters is the absolute angle of attack of the wing. If this value is positive, lift will be produced and level flight is possible, whatever the aircraft attitude.

You might add something about angle of incidence in case the book is referring to airframe AoA

And talking about airframe AoA is rather common, because with wash-out, the wing AoA is not all that well defined.

Yes, with fuselage angle of attack zero. It makes a lot of sense to design the aircraft such that at cruise speed, the fuselage is aligned with the free stream so that there is minimal drag. The wing Mean Aerodynamic Chord then usually has an incidence to support enough lift to support aircraft weight, although if the wing has plenty camber, it is also possible to fly with zero MAC angle of attack.


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N registration in Europe
Do someone knows a way how to arrange an American(N) registration of aircraft located in Europe? I mean there should be an agency that you can entrust with carrying out an American registration of an aircraft that is in Europe. I am looking for this office.

It isn't difficult, but you have to meet certain requirements. Namely you need to be a citizen of the united States or a US company.

@RonBeyer, surprisingly N55XZ appears to be registered by Austrian company, not a US one. The plane is based in Czechia and I doubt it ever operates outside Europe.

@JanHudec Looking at some records for that aircraft, it seems that the N number was registered originally to an address in Houston, TX. Maybe that is how they did it...

@RonBeyer, that would mean when you buy an airplane that has valid N registration, you can keep it even if you don't otherwise satisfy the conditions...

The FAA handles all US aircraft registration:  FAA Aircraft Registration  Note that there are ownership requirements related to US citizenship etc.  See FAR 47.3


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What are the factors that determine the speed flutter appears on wings?
I'm doing a study on the flutter phenomenon of airplane wings. I'm interested in factors that influence wing stability, and how one can increase the speed where flutter starts.
What are the parameters that determine this speed?

Welcome to Aviation.stackexchange saud. Nice first question.

It's an aeroelastical phenomenon driven by pitching moment, torsion stiffness, and inertia of the wing. It's a classical spring-mass-damper problem: increased aerodynamic pitching moment causes torsion on the wing structure, which causes more pitching moment until the local airfoil AoA is such that the air flow separates. Torsion is counteracted by the wing torsional stiffness, thin wings have less torsional stiffness.
Parameters that determine flutter speed:

Wing profile: aerodynamic $C_m$ and wing thickness
Wing aspect ratio
Number of jet engines mounted forward of the aeroelastic axis
Wing sweep

And, for propeller aircraft, propeller RPM and engine-mount stiffness, as Lockheed found out to their sorrow with the Electra.


text:
How would I read a departure chart?
How do I read charts like these?
I don't know how to read it, or where to get started on IFR departures and approaches. I can talk to ATC and fly VFR, but I am still unable to fly IFR and would really like to start.
What else do I need to know to start on IFR?
What are fixes, airways, and routes? How do I plan a route myself?

Welcome Justin, have you tried an IFR tutorial? Or studied documents about SIDs from the same source. For extreme aviation enthusiasts, there are books like this one which make the bridge between game and real world.

So there's a lot going on in your question to address. First, and most importantly, the info you linked is for ATC, not pilots. you won't get any usable info of how to fly IFR in this airspace. You won't need to know any of this to fly IFR. You won't need any of this to fly VFR either, for what it's worth.
When you fly IFR you file a flight plan that includes your route, destination, and nav points, which are those funny named things like WAVEY in the chart you linked. This is done before you even step in to the plane. Generally a day ahead or early in the morning of your flight, but you can file it right before. a lot of pilots (this is for non-commercial) will take off VFR and then "pick up" their filed flight plan after they are en route. this means you don't have to wait for an IFR release on departure. You just take off, fly in the general direction you are headed, and then ATC will direct you via nav points the rest of the way (or if you're lucky tell you to proceed "as filed", which means no changes to your original plan). For commercial they don't usually have to wait for the release because they are scheduled flights. Thats kind of a different world. I bring up how it happens for non-commercial because you will do a LOT of that kind of flying before you have to be worried about how United and Delta get off the ground.
an IFR approach plate for pilots is still confusing, but there isn't nearly as much going on as what you have linked. Approaches are very very specific for the runway, type of approach (look up types of instrument approaches), and a number of other factors. specific to the point of each has a special thing you do if you can't land for any reason and have to "go missed". This is a huge portion of the IFR training, and nobody can really be self taught this or know how to do it right away. IFR is the hardest rating for a reason.
Don't get too cocky about being able to talk to ATC while VFR. Thousands of experienced pilots a year go through comms training because it's hard. In vatsim is one thing, but the real world of radio is one of the most stressful and elusive parts of the flight. get something wrong and you can lose your license or your life.
If you want to get started with reading charts go to sportys.com and buy their VFR sectional training chart. It's 3 dollars and gets you a manageable section of a navigational chart to learn how to plan and plot and read an aeronautical chart. Theres a lot going on there, you have true longitude and magnetic longitude lines, VORs, airport info, waypoints, obstructions, ground levels, airspace, MOAs. Don't think of VFR as "the easy flying".


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How does radar detect turbulence and wind speed?
In an answer to a questions about turbulence this answer mentioned that aircraft use radar to help detect the most turbulent areas of a storm so that pilots can avoid them.
How does radar do this?  Does it have to do with rain inside the storm moving up and down with the wind?  And if so, can radar detect wind speeds outside of a storm at all?

Have you read the answers here?

@fooot  Have now, but neither of those answers really answer my question.  They give a good general overview of weather radar, but don't really give detail on how weather radar see's turbulence.

More on CAT detection by airborne UV lidar: DELICAT LIDAR system alerts pilots to air turbulence ahead.

@mins You might consider spinning that into an answer and seeing how everyone votes on it.

More on Low Level Wind Shear detection on airport in Japan by lidar and radar combined. Also this presentation.

@JayCarr: I wish I could provide a complete summary, however I don't have a good background on the topic, I just recall some readings about lidar. I hope someone will be able to include these elements in a full answer.

The answer you are referring to explains how radar is used to fly through a squall line whilst avoiding the most turbulent areas.
Conventional weather radar cannot detect wind speed or turbulence directly, it only detects solid and liquid objects above a threshold size.
The most turbulent areas of thunderstorms are also those areas that contain the most water and ice. This precipitation reflects radar waves very well so the most turbulent areas have the strongest reflections and shows up as red / purple on the scope. The area downwind of the core is usually very turbulent as well.
So the weather radar is merely detecting the presence of water. It is the pilot interpreting the picture who is really detecting the turbulence.
Clear air turbulence  (CAT) does not have condensated water in it and is therefore undetectable by conventional radar. There are systems such as LIDAR that can detect CAT but they generally not installed on aircraft.
As @reirab noted in a comment, a Doppler radar can be used to detect the relative speed of the particles from an aircraft. This is not used to reliably detect wind speed, but when there are large variations in wind speed (a.k.a. turbulence) the Doppler shift in the reflections from the particles causes a spectrum spread. This is an indication for turbulence. The traditional S-band airborne weather radars do not have this capability but the newer X-band radar (e.g. this Honeywell radar (PDF)) can detect wet turbulence to distance of up to 40-60 nautical miles using this technique.

LIDAR (a portmanteau of Light and RADAR) is a technology that uses light instead of radio waves. LIDAR can measure position and relative velocity of aerosols and therefore it can be used to detect turbulence. It has been used successfully to detect wind shears and map wake vortices on final approach from the ground (paperPDF).
Currently investigation (paperPDF, presentationPDF)is going on into airborne use of LIDAR to detect CAT. One of the difficulty is to detect vertical components of the turbulence. This causes very little Doppler shift but at the same time have the most effect on the aircraft (since the vertical component of turbulence causes changes in angle of attack).
As far as I know there are currently no commercial applications of LIDAR for airborne turbulence detection.

Conventional radar can't detect wind directly, but it can detect the motion of particulates (like water droplets) via the Doppler effect (which is why weather radars are often called 'Doppler radars.') Since the velocity of the water droplets will usually be more or less equal to the local wind velocity, we can figure out wind speed (at least the component of it toward or away from the radar) using this effect. You're right, though, that this is harder for CAT, due to the lesser magnitude reflections from particulates.

@reirab You can't reliably make out the wind component from an airborne weather radar. When the Doppler shift causes a spread spectrum you can detect variation in wind speed when there is wet turbulence. This is used on the newer generation X-band weather radars. Many aircraft still fly around with the more conventional S-band weather radar, which doesn't have this turbulence mode. The X-band types can detect wet turbulence to up to a range of about 40-60 NM.

@reirab I don't think typical weather radars do anything to measure these velocities though, or at least they are not displayed.  Radar displays normally show "base reflectivity" in dBm. This has nothing to do with the velocity of the particles doing the reflecting.

@JoelM. I'm not sure about airborne radars, but it's done all the time on land-based weather radars. In the U.S., the National Weather Service makes graphical displays of the wind data available to the public from all of its Nexrad radars. See here, for example. This is especially useful for detecting high levels of wind shear, which can indicate rotation (and, therefore, tornado formation.) This is one of the NWS' primary tools for issuing tornado warnings.

@JoelM. Note that what is being measured here is different. For wind speeds, they're measuring Doppler shift, i.e. the frequency of the received signal (and its difference from the frequency that was originally transmitted.) It's the amplitude of the received signal that is displayed in dBm, which gives you information about precipitation intensity. Both the frequency and amplitude information are derived from the same received signal, though.

I know they aren't found n airplanes, but could you perhaps explain a bit about what lidar is, how it detects CAT and why they don't have it on planes.  I'm hoping it will help contextualize casey's answer if you put that in yours.

@reirab I understand the measurements made (I work on radar), just didn't think they displayed any of the Doppler data on sites like NWS. It's interesting to see that they do.

@JayCarr LIDAR is typically too large, complex, heavy, expensive etc to install on anything but a dedicated sensor aircraft.  The LIDAR's that they use to measure CAT are also typically ground-based, I believe.  Don't know a lot of the details for how they detect CAT but it's based on changes in refraction in the atmosphere that are present in areas of turbulence.  That's going to be very difficult to do from a moving platform, if not impossible.

@JoelM. Perhaps you could edit that into the answer?  I know it's not something that benefits you directly, but it would be nice to have in the actual answer...  I'm also still not certain how LIDAR works?  Does it use a different frequence so it can be more granular in it's readings?

@JayCarr I'll add lidar info tomorrow, currently too busy with other urgent ATM matters.

@DeltaLima Sweet!  Thanks, looking forward to reading it :)

@JayCarr It took a bit longer than anticipated, but LIDAR is added.

The short answer is that radar cannot directly observe the wind.  What radar can detect is the velocities of small particles lofted into the air.  This is done by measuring the doppler shift of the energy returned to the radar.  The radar can only detect the component of velocity toward or away from the radar.
The radar doesn't have to diagnose turbulence within convection, as that is assumed to be there already and is part of the nature of convection.  It is much more valuable to detect clear air turbulence which does not have the visual indications that boiling cumulus growth does.
Thankfully, clear air generally isn't exactly clear.  If there are aerosols or bugs that are large enough to scatter radar energy then the radar can infer their motion as the wind's motion.  For small things like dust and insects this isn't a bad approximation.  The return energy of these scatterers is very small and is generally below the threshold that the radar display will plot (unlike birds or bats), but the computers can interpret the data.
The detection of turbulence is algorithmic, and while I do not have knowledge of specific algorithms used, my first order guess is that they look at the variation in doppler velocity with distance from the airplane and they look at the time-series of doppler velocities at each sampled location in subsequent scans.  If the variation meets some criteria this could be classed as "turbulence" and provide an alert to the pilots.
The ability of such a system also depends on the physical capabilities of the radar including the wavelength the radar operates at, its sampling resolution and the sampling rate.

Radar actually detects clear air turbulence using changes in refractivity caused by turbulence, not the motion of particulates.  No radar that I know of can see and measure the motion of objects as small as bugs, they are effectively invisible to radars of any wavelength large enough to propagate in the atmosphere.  Measurement of refractivity is very difficult to do, however, and as far as I know there are very few radars that are actually designed to do it.  See: http://journals.ametsoc.org/doi/abs/10.1175/1520-0450%281966%29005%3C0450%3AOTRDOC%3E2.0.CO%3B2

Effectively what I'm saying is that CAT(clear air turbulence) is not typically measured with radar.  It is inferred from things like convective weather and precipitation, but not measured.

@JoelM. I welcome an edit to my post if you want to contribute your knowledge.  As far as bugs, I can't attest to airborne radar, but I think you kind find plenty of examples on S-band e.g. the WSR88D network.  I've also seen similar use of lofted insects causing radar fine lines at convergence boundaries both on WSR-88D and the various incarnations of the DOW vehicles.  There has even been some recent research into the dual-pol signatures of insects, e.g. http://journals.ametsoc.org/doi/abs/10.1175/JTECH-D-13-00247.1

it sounds like your experience in this area is greater than mine, so I can only hypothesize but I do know that radar will only be able to detect things like bugs and particulates in large, dense quantities.  So while I suppose it's possible to detect turbulence in that way, it doesn't seem like a very reliable method.

Do you know of any radar system that is regularly used to directly detect turbulence?  The only ones I've heard of using the refractivity method are ones built for other purposes, and then used to detect CAT for research.

@JoelM. I don't unfortunately but it wouldn't surprise me of some of the research radars I've seen are  capable. It might be a good question to ask on [earthscience.se].

So...  Is this a system that is currently in aircraft?  And is this answer correct?  I'm having a hard time figure it out after reading the comments.


text:
Is someone holding an EASA PPL (A) allowed to operate a "November" registered aircraft in Europe?
Is someone who holds a current EASA PPL(A) allowed to operate an FAA "November" registered aircraft in Europe? Of course I'm talking about a type of aircraft and of operation he or she would be allowed to conduct with an, for example, German-registered aircraft.

Hehe, that's the reason why I'm asking :D

You're specifically referring to Germany in your question. Is your question limited to Germany or anywhere in Europe?

Thanks for your edit Philippe. I'm a German, but in this question Germany is only used as an example. I was not happy with replacing this last part with the word operation, but if you like we can use Europe instead of Germany.

This is regulated by the FAA in FAR 61.3 and local regulations specific to each country

(a) Required pilot certificate for operating a civil aircraft of the
  United States. No person may serve as a required pilot flight
  crewmember of a civil aircraft of the United States, unless that
  person:
(1) Has in the person's physical possession or readily accessible in
  the aircraft when exercising the privileges of that pilot certificate
  or authorization—
(i) A pilot certificate issued under this part and in accordance with
  § 61.19; (ii) A special purpose pilot authorization issued under §
  61.77;
(iii) A temporary certificate issued under § 61.17;
(iv) A document conveying temporary authority to exercise certificate
  privileges issued by the Airmen Certification Branch under § 61.29(e);
  or
(v) When operating an aircraft within a foreign country, a pilot
  license issued by that country may be used.

To operate a US-registered aircraft you either need an FAA license or a pilot license issued by the country where the aircraft is operated in. This also means that you are not allowed to fly into another country, unless you also have a license issued by that country. Even if individual countries allow foreign registered aircraft to be operated by a license holder of another (non-FAA) country, it would still be a violation of FAR 61.3.
Bottom line: if you want to fly a N-registered aircraft without an FAA pilot license in Europe, you need a pilot license issued by every country you're flying into. For example, if you plan a trip from The Netherlands to Portugal you would need 5 different EASA PPL licenses issued by The Netherlands, Belgium, France, Spain and Portugal.

I would agree, the whole purpose of EASA issued licenses was to have one license across Europe. I'd be surprised if you couldn't take an N-Reg from Germany to the Netherlands with a PPL-A EU-FCL.

@SentryRaven Sadly, that's not how it works, whether it makes sense or not. The FAA regulations are pretty clear about that.

This answer is not correct. As @Qantas94Heavy pointed out, the text of (a)(v) says "may." The FAA does not require a foreign license. FAA-certificated pilots routinely fly FAA-registered aircraft through, into, and back from (to name one particularly common example) Canada, and do not require a Canadian pilot's license. I'm not as familiar with EASA regulations, but I very much doubt that a license from each of the countries in your last paragraph is required in your example (and certainly would not be required by the FAA).

@dvnrrs The answer is correct. I said that you "either need an FAA license or a pilot license issued by the country". So if you don't hold an FAA license, you need a license of the country you're flying in. In your example about Canada: the pilot has an FAA license (as you specifically said), so there's no issue. I will clarify that again in the last paragraph.

@PhilippeLeybaert Because EASA didn't exist when 61.3 was written I think this is a case where the wording of the regulations don't seem to match their intent (to the extent that I'm actually going to draft a letter the FAA counsel's office and see what they have to say). I would interpret 61.3 to mean that "An EASA license is valid in any country that issues/recognizes EASA licenses", otherwise the EASA license is kind of useless - but while I think that's what it means as you point out that's not what it says.

@PhilippeLeybaert NO, the answer is not correct. There is no requirement to have a Dutch license for flying a US registered aircraft in the Netherlands for exemple. A US license suffices. This is different from flying a Dutch registered aircraft, this is illegal in the Netherlands with a US license. Which is where your confusion probably stems from. The Netherlands for that reason contains a number of US registered aircraft, specifically for rental to people who have a US license and thus can't rent a Dutch registered aircraft.

in fact, if your reasoning is correct, it would be illegal for anyone to fly in any country other than the one he got his license without taking flying lessons there first and getting a local license. This is patently silly of course.

@jwentling Did you actually read the question and my answer? It's about flying an N-REGISTERED aircraft WITHOUT A US PILOT LICENSE in Europe. "WITHOUT". I specifically said that you need either an FAA license (U.S. license) or a license issued by the country you're flying in.

@jwenting; the (correct I might add) reasoning is that you can take your US license and fly an N-registered aircraft anywhere you want (where your license is accepted). However, you can't take a Chinese or Russian license and fly an N-registered aircraft anywhere but within China or Russia. I don't know for certain if you're allowed to fly an N-registered aircraft throughout the EASA-area or not on an EASA license though. It used to be like is described here, but it might have changed since EASA got underway. I don't know if you can even have more than one EASA license.

@PhilippeLeybaert Just to follow up, I heard back from the FAA and they've confirmed that your interpretation is correct - 61.3 and the Chicago Convention render the EASA license invalid once you leave the issuing country (unless the state of registration of the aircraft & the state issuing the license are the same). That could mean collecting a lot of licenses in Europe if you're flying an N-registered aircraft :-)

The solution you are proposing with multiple national licences is problematic, as EU1178/2011 FCL.015 c) states: "A person shall not hold at any time more than one licence per category of aircraft issued in accordance with this Part." effectively barring from holding EASA Part-FCL licences in multiple countries. Similarily JAR-FCL 1.065 e) states/stated: "An applicant shall hold only one JAR– FCL licence (aeroplane) and only one medical certificate at any time."

Regarding EASA regulation 216/2008 and 1178/2011 (EU).  Since April 8, 2015, the EU laws also require pilots who live in Europe, and fly a foreign registered aircraft (for example an N-registered aircraft) that they also have to have the EASA license.
There will be a postponement of this regulation by one year, i.e. until April 8, 2016, that the EU intends to publish in the coming weeks.
Until 2016, there will be a bilateral agreement between the EU and the USA, providing a simple mutual recognition of licenses, stand-alone licenses.
This will make it much easier to get a FAA stand-alone license.

do you have any links to the actual regulations you are mentioning?

EU 1178/2011 http://easa.europa.eu/document-library/regulations/commission-regulation-eu-no-11782011 EU216/2008 https://easa.europa.eu/document-library/regulations/regulation-ec-no-2162008

Can you please explain how it would change the process of getting a FAA license if you already have a valid European PPL license? As far as I know, the new EASA regulations won't make it any easier to get a standalone FAA license for European pilots.


text:
What is the pilot's obligation to verify a registration has not expired?
Old registrations had no expiration date, so a pilot could get in a plane that is owned by someone else and verify the airworthiness and registration by inspecting the documents. But that registration might be expired, and without going to the FAA site to verify it is active could find themselves flying an unregistered aircraft. Has the FAA addressed this situation since they started expiring registrations that didn't originally expire? What would the pilot's liability be in this situation?

You verify new (expiring) registrations the same way you verified the old ("for life") registrations -- by looking at the document. You just have to pay attention now (it's not just "It's there", you need to check the expiration date too).
New-style registration certificates (issued some time after October 2010) are supposed to have the expiration date clearly printed on the certificate (In the lower-left corner, under the date of issue).
For certificates without an expiration date printed on them (registered prior to October 2010) the FAA established a table of expiration dates back when the re-registration requirement was initiated.
ALL of the old certificates have expired by now: The last ones expired December 31, 2013.

As regards the pilot's liability, an unregistered aircraft (or one with an expired airworthiness certificate) is de facto unairworthy:

Does expiration of registration affect an aircraft's airworthiness certificate?
Because an aircraft with an expired registration is not registered, its airworthiness certification would be considered ineffective. Without registration the aircraft is not authorized for flight.
…

From The FAA's registration FAQ.
Aside from being a violation of the FARs to fly the aircraft with an expired registration (because it's "not airworthy") this triggers other implications, for example most aircraft insurance is invalid if the aircraft is "not airworthy" .

You mean de jure unairworthy - unworthy because of law. De facto it might be airworthy and completely safe to fly, even if flying it would not be legal.

91.203 and 91.403 make it pretty clear that the owner or operator is primarily responsible for maintaining the aircraft in an airworthy state. If you are flying it, you are the operator and it is up to you to ensure that the plane is registered and airworthy.
You should never get in a plane and fly it without verifying that its maintenance, airworthiness, and registration are up to spec. It's not difficult.
As for checking registration itself, simply check if the document is more than 3 years old. If it is, it is most likely expired.


text:
What is the fastest airplane?
There are many airplanes that claim to be the fastest piston, private jet, airliner, but what is the all time fastest airplane? How fast does it travel?

Sorry, I know the question kind of asks for "one" "all-time fastest, period" airplane, and my complicated answer gives a ton. If you want to provide specific definitions for "aircraft" and specify method of propulsion (namely, do rockets count) and any other restrictions (see my question) then I can probably give you one single definitive "fastest ever" answer :)

Thanks for the comprehensive answer. The question was for airplane not aircraft, which according to wikipedia is a fixed wing aircraft powered by a jet or propellor.

Somewhat related, due to the discussion of "relative to what?" is this xkcd comic about what object has traveled the furthest (which thus requires calculations of how fast things travel).  It isn't specifically about aircraft, but still.

what do you consider to be an airplane?

This is a broad question. The Wikipedia page on vehicle speed records gives a nice breakdown by various categories. Some criteria are manned vs. unmanned, air-breathing vs. rocket, civilian vs. military.
The fastest manned, air-breathing aircraft was the Lockheed SR-71 Blackbird with an (unclassified) record of 2,193.2 mph on July 28, 1976.
However, some would consider the rocket-powered North American X-15 the fastest manned aircraft at 4,510 mph.
If you consider the Space Shuttle an "aircraft" during its re-entry phase, it passed Entry Interface at around 17,500 mph. This was unpowered flight though, and its speed dropped off as it entered the atmosphere.
I believe the rocket-boosted HTV-2 Falcon currently qualifies as the fastest unmanned aircraft at 13,201 mph.
The record for the fastest human-made craft of any kind is currently held by the Helios probes, at 157,078 mph. The fastest manned human-made craft was the Apollo 10 spacecraft at 24,790 mph. (Note that Apollo 10 holds the record not because it was any more or less capable than any other Apollo mission, but that its chosen moon fly-by trajectory resulted in a higher top speed relative to Earth. It could also have achieved a much higher top speed on a different trajectory had that been a goal for mission planners, but it wasn't.)
Finally, every single one of these is either military or NASA (not civilian). The fastest civilian, air-breathing aircraft were the Tupolev Tu-144 supersonic transport at 1,518 mph, and its Western counterpart, the legendary Aérospatiale-BAC Concorde, at 1,488 mph. Both are now retired. Several private organizations are developing civilian suborbital and orbital rocket-powered spacecraft; I'm not sure which is currently fastest.
And though these aren't crafts or vehicles, honorable mentions go to a 150,000 mph steel plate launched by a nuclear weapon, and to particle accelerators for propelling subatomic particles and "man-made" particle collision byproducts at ridiculously close to (99.99something percent of) the speed of light :-)

SpaceShipTwo (or it's predecessor, SpaceShipOne; I am not sure whether SpaceShipTwo already flew the full planned profile) is civilian and faster than Concorde. It is rocket-powered though, so it leaves Concorde as fastest air-breathing civilian aircraft.

I wonder what the fastest plane designed for private usage is...  Since Concorde was commercial and all.

@JayCarr Possibly the Cessna Citation X at Mach 0.935.

Voyager 1 beats Juno - 3.6 AU / year (or about 38,610 mph) http://voyager.jpl.nasa.gov/mission/fastfacts.html , but the fastest man-made object was a piece of steel launched at over 147,000 mph http://en.wikipedia.org/wiki/Operation_Plumbbob#Propulsion_of_steel_plate_cap

@dvnrrs That's right, because civilians aren't allowed to have planes that go super-sonic, are they?

IF Juno reaches Jupiter, it is projected to reach speeds over 150,000 mph, but that won't happen until sometime in 2016

@DavidWilkins Touché, I should have said "craft" instead of "object." But in that case I'll put in an argument for the LHC (technically the particle collision byproducts are "manmade") :-) And thanks for the correction re: Juno. I thought it had already exceeded Voyager's speed.

@JayCarr Not exactly. At least one supersonic MiG-29 is privately owned by a civilian but I'm not sure if it's been rendered incapable of supersonic flight or whether it can still be legally operated supersonically. Certainly supersonic flight over the continental US is banned (except in certain military areas).

@dvnrrs Likewise, nice find with Helios, I was unaware.  And it's probably best to not include manmade particles...Every time we turn on our cellphones, light a match, etc.  We're "creating" photons that travel at c

@DavidWilkins I vote that photons be disqualified since they have zero rest mass. But yes, we are definitely stretching the scope of the original question (not to mention reasonableness) now :-)

@JayCarr commercial is private :)

@jwenting - I dunno, there are a lot of things companies are allowed to own that individual people are not...  That's what I was getting at.

BTW- The protons in the LHC are firmly beaten by the electrons in LEP II (the machine that used to occupy the tunnel the LHC is in) and both are whooped by various neutrino beams in Europe, America and Japan. Not that particle beams are that interesting in this discussion, but just for completeness.

The Tupolev Tu-144, which saw brief civilian passenger service in the USSR, was faster than Concorde.

At present (and since 1976) Lockheed SR-71 Blackbird holds record for the fastest aircraft. As mentioned here:

The SR-71 was the world's fastest and highest-flying operational
  manned aircraft throughout its career. On 28 July 1976, SR-71 serial
  number 61-7962, broke the world record: an "absolute altitude record"
  of 85,069 feet (25,929 m).

The speed it traveled was 1,905.81 knots (2,193.2 mph; 3,529.6 km/h), approximately Mach 3.3.

A list of airspeed record is on Wikipedia.

SR-71 = fastest manned, air-breathing aircraft. Important distinction.

Also, keep in mind, these are only declassified pieces of information...  For all we know there's some aircraft out there that is significantly faster.

@JayCarr I don't know that. I am not told any classified information.

@Farhan - lol, it's just an observation.  If you had access to classified information, I certainly hope you wouldn't share it here ;).

@JayCarr if he had classified information he'd not even be able to share that he has classified information...

@jwenting I do have some classified information, but I will not share it because it is called personal information. :)

It's pretty unlikely that the U.S. military ever had a manned air breather faster than the SR-71.

@JimInTexas Not yet, anyway.

The NASA X-43A got up to almost Mach 10, I think that's pretty close to a record for an air breathing atmospheric vehicle.


text:
What is the minimum ceiling and visibility limits in Class E airspace?
With the intention of landing, taking off, or entering the traffic pattern at an airport in Class E airspace, what is the minimum ceiling and visibility as a VFR pilot in the USA.

Can't you easily find that online?

VFR cloud clearance requirements are listed in 14 CFR 91.155 and for Class E airspace specifies:

Class E:
Less than 10,000 feet MSL.
  Flight Visibility: 3 statute miles
  Distance From Clouds: 500 feet below, 1,000 feet above, 2,000 feet
  horizontal.
At or above 10,000 feet MSL.
  Flight Visibility: 5 statute miles
  Distance From Clouds: 1,000 feet below, 1,000 feet above, 1 statute
  mile horizontal.

There is also a requirement specific to Class E surface areas that says that the ceiling must be at least 1,000 feet in order to operate in the airspace.  It is also listed in 91.155:

(c) Except as provided in Sec. 91.157, no person may operate an
  aircraft beneath the ceiling under VFR within the lateral boundaries
  of controlled airspace designated to the surface for an airport when
  the ceiling is less than 1,000 feet.

Note that there are exceptions for both of these that refer to 91.157 Special VFR Weather Minimums if you are instrument rated and the aircraft is instrument equipped.

Given a scenario of clouds being at exactly 1,000 ft AGL over a Class E surface area airspace, would it be correct to say that the pilot operating under VFR would have to fly the pattern at no greater than 500 ft AGL to keep cloud clearance per class E requirements?

@RyanGriffith That's correct!

That's sure to piss the neighbors off.